1. Field of the Invention
This invention relates to electrical devices of the type having a contact brush in electrical engagement with, and relatively movable with respect to, a coil wound upon a magnetic core and mounted upon a base, the movement of which brush varies the output of the device, and more particularly to means for dissipating heat generated in the coil.
2. Description of the Prior Art
Generally, the maximum power that such a device can handle is limited by the temperature rating of the hottest section of the coil. Exceeding this temperature rating can result in fire and/or electrical shortcircuiting. The temperature of the coil is not necessarily uniform and maximum temperatures are produced when the device is operated such that a large current is carried in only a relatively small portion of the coil and a much smaller current is carried in the balance of the coil. This condition typically becomes most extreme when the output voltage of the device is near the input voltage. Since, among other things, the temperature of a wire is approximately proportional to the square of the current it carries, the temperature of the small portion of the coil carrying the large current can be considerably greater than that of the balance of the winding which is carrying the smaller current.
Although the core and the base provide paths for the conduction of heat to the air surrounding the device, their effectiveness in conventional devices is limited. One limitation is a layer of electrical insulation separating the coil from the core, which insulation is usually a relatively efficient thermal insulator. Another limitation is the electrical material typically placed between the coil and the base, which insulation is also usually a relatively efficient thermal insulator. Yet another limitation is the relatively small surface area of the base which limits the quantity of heat that can be transferred from the base to the surrounding air. Likewise, the relatively small surface area of the hot portion of the coil is a further limitation. The net result of these limitations is that the heat generated in the coil has no efficient path for dissipation and a device of a given size can handle less power than it could otherwise if the effect of these limitations could be reduced.
Some prior art designs place the device in a bath of cooling oil to increase heat transfer, but this has the drawback of relatively higher cost and can cause environmental problems.